Our work focuses on the use of DNA-based vaccine strategies both as preventive and immunotherapeutic approaches. We have generated efficient SIV and HIV DNA expression vectors. This work is based on our previous recognition that RNA elements (called INS) present within the gag/pol and env coding regions of HIV are responsible for nuclear retention and instability of the transcripts in the absence of Rev, and that these elements can be eliminated by changing the nucleotide composition of the transcripts without affecting the amino acid sequence. Immunogencity of the antigens was further improved by modifying the trafficking of the antigens. The introduced modifications of the proteins led to more efficient secretion of the SIV antigens resulting in increased cellular and humoral immune responses in the vaccinated mice or rhesus macaques. Using DNA-only vaccination we showed that our optimized DNA vaccine vectors are able to induce potent immune responses able to protect from high viremia in the rhesus macaque/SIVmac251 model. In collaboration with other investigators, we have shown that these DNAs provide an excellent prime in DNA prime-recombinant virus studies inducing protective immune responses. We investigated the contribution of cytokines co-administered as molecular adjuvant to further improve the efficacy of DNA vaccination. We found that the use IL-12 and IL-15 plasmid DNAs as adjuvants resulted in greatly improved immunogenicity in mice and macaques. To further improve immunogenicity, we used alternative DNA delivery systems and showed that DNA delivery via in vivo electroporation elicits greatly improved cellular immune responses in rhesus macaques. Immune responses were of great magnitude, long-lasting and broad. Importantly, we found great induction of not only systemic but also mucosal cellular and humoral immune responses. In a recent preventive vaccination study, we demonstrated that improved gene delivery and expression dramatically improves immunogenicity and effectiveness of DNA vaccination. We have investigated the effect of DNA vaccination during antiretroviral treatment (ART). We have found that vaccination of SIV-infected rhesus macaques induces potent immune responses that are able to control viremia after ART interruption. Thus, DNA vaccination has great potential to be used as an additional therapeutic modality. We have found that DNA vaccination can be applied repeatedly leading to further control of viremia. To design better vaccine strategies, it is important to dissect correlates of protective immunity. Among the 'controllers'are animals from our vaccine studies as well as animals infected with live-attenuated SIV strains, which serve as model to study the underlying mechanisms of protection from disease development. Such animals allow us to dissect the cellular and viral determinants that contribute to disease development and they also provide a unique resources to study mechanisms leading to protective immunity. These studies will provide critical information about the establishment and maintenance of host immune responses during chronic retroviral infections with distinct pathogenic outcomes and will facilitate further improvements of DNA vaccines.